Gypsum board is well known and widely used in the construction industry as a convenient way to construct walls, barriers and other structural formations. The use of inorganic gypsum board, which is also commonly known as “wallboard” or “drywall,” is often desirable over more expensive and time consuming conventional wet plaster methods. A typical sheet of wallboard comprises a gypsum core, a back cover sheet on one surface of the core and a face or front cover sheet on the other core surface. One cover sheet is typically folded around the long side edges of the core and overlaps the side edges of the other cover sheet. Apparatuses and methods for the commercial manufacture of wallboard are well known, and instances of such apparatuses and methods can be found, for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, Second Edition, 1970, Vol. 21, pages 621-24, which is incorporated herein by reference. Other examples can also be found in U.S. Pat. Nos. 5,718,797 and 5,879,446, both of which are commonly assigned to the assignee of the present application, and both of which are incorporated herein by reference in their entirety.
Generally, wallboard is conventionally produced by sandwiching a core of aqueous “slurry” or paste of calcined gypsum and other materials between two extremely long and continuous sheets of board cover paper. Various types of cover paper are known in the art, and all such types can be used for this purpose. After the gypsum slurry has set (i.e., reacted with the water from the aqueous slurry) for a period of time, the resulting extremely long board is then cut into manageable sections. These sections are then fully dried and the finished product becomes a strong, rigid, fire-resistant building material, which can then be cut into various board sizes as desired.
Commercial manufacture of gypsum wallboard is often accomplished by processes that are capable of operation under continuous high speed conditions. In such applications, the aqueous slurry of calcined gypsum and other ingredients are continuously deposited onto a first continuously supplied and lengthy moving sheet of cover paper. Shortly thereafter, a second continuously supplied and lengthy moving sheet of cover paper is then directed over the top of the slurry, such that the slurry is then sandwiched between the two sheets of cover paper. This resulting product continues to move onward from this “wetend” location (where the paper and slurry are combined) at high speeds.
Because continuous high speed operation without interruption is desirable in the manufacturing process, “on the fly” splicing of a new roll of cover paper to the end or near the end of a depleted roll is common practice. It is thus typical to have a pair of adjacent spindles for mounting two huge rolls of cover paper for both the first and the second continuously supplied and moving sheets of cover paper. A machine or operator monitors a roll of cover paper in use and notes when that roll of cover paper is close to expiring. A leading edge of the adjacent new roll of cover paper is primed, and at an appropriate time a “splice bar” is usually employed to quickly and uniformly attach this leading edge to the depleted roll via appropriate attaching means.
Such a splice bar is well known in the art, and may be automated or manually controlled. In practice, the cover papers are continuously moving and the spindles are rotating at high enough speeds such that a splice is made some distance in front of the trailing edge of the depleted roll. Because overlapping or double layering of cover sheets is particularly undesirable, as discussed in more detail below, the remainder of the depleted roll is then cut off just behind the splice after the splice is made. Even the best splicing process, however, results in at least some short segment of overlap between the old and the new cover papers where they are attached to each other. The frequency of this problem is doubled due to the need for splicing in both the top and bottom cover papers.
Continuous movement away from the wetend location where the slurry and paper come together typically takes place on multiple conveyors, rollers, or other similar devices laid forth in series, such that the product being manufactured generally comprises a continuous piece of setting wallboard that extends for hundreds or even thousands of feet. Although lengths, speeds and times may vary in this type of wallboard manufacturing process, a long “board line” is needed in order for the slurry to set for a sufficient amount of time before cutting can be attempted. This setting time should be anywhere from 3 to 6 minutes, with board line lengths and processing speeds varying to ensure that a minimum setting time is met. Of course, the actual lengths and speeds may be adjusted as desired to control for not only an appropriate amount of time for the slurry to set, but also to increase the amount of wallboard that can be produced.
Initial cutting of the resulting continuously formed sheet of wallboard is typically accomplished through a machine such as a rotary knife. When activated, such a knife rotates as it cuts the rapidly passing wallboard, such that a clean cut is made and the wallboard is not buckled, sheared or otherwise significantly deformed at the cut edges. A processor or other type of control unit can be set up to control this knife to some degree with additional manual controls and input also being available. This rotary knife initially cuts the continuously moving wallboard into large but manageable sections. These sections are then processed on different conveyors or rollers through layered kilns in order for the sections of cut wallboard to fully dry and harden to a final state before they can be cut into smaller, commercial standard sized pieces. Drying in heated kilns, however, tends to warp, buckle, pop, cavitate, crumble or otherwise distort wallboard due to uneven drying at any imperfection or exposed edge. As a result, cut sections of wallboard are typically sent through the drying kilns side-by-side and end to end in order to minimize exposed edges and the corresponding amount of distortion due to uneven drying. Thus, wallboard sections are often put into pairs or otherwise grouped after cutting and before kiln drying. This process of pairing or grouping wallboard sections can also be automated, and often involves some sections of wallboard being flipped over from the end of the cutting conveyor belt or roller onto the start of a drying conveyor belt or roller.
Cover paper splices, as described above, are a particularly undesirable imperfection in wallboard, and prudent manufacturing practices dictate against sending any section of wallboard containing a splice through the drying kilns. In addition to resulting in an ultimately poor commercial product, wallboard sections containing a cover paper splice are particularly susceptible to distortion and/or disintegration while in a drying kiln. Such wallboard disintegration can sometimes result in an unwanted system shutdown in order to clean and restart a jammed or contaminated kiln. Accordingly, wallboard segments containing a cover paper splice are routinely isolated and rejected after they are initially cut by a rotary knife and before they are processed through a kiln. Although most of the materials in a rejected wallboard segment can be recycled, excessive scrapping and/or recycling of rejected materials results in reduced productivity and introduces undesirable strain and wear on the manufacturing system. As such, the necessary removal of wallboard sections containing cover sheet splices is considered to be waste regardless of whether some material from such sections is eventually recycled.
Furthermore, because wallboard sections tend to be kiln dried in groups of two or more, the removal of one wallboard section due to a problem or defect such as a cover paper splice usually means that at least one other satisfactory wallboard section must be removed and go to waste as well. This is particularly true where a substantial portion of the overall process is automated, such that removal of one defective section of wallboard leaves the preceding or following section without a mate for the grouping and drying process, absent manual intervention. Although this results in a significant amount of wasted product every time a cover paper splice reaches the knife, such waste is a matter of course in most commercial wallboard manufacturing operations.
Compounding the issue is the fact that the rotary knife is typically automated, and is usually set to make repeated cuts only at standard intervals or distance increments that result in repeated sections of cut wallboard of a particularly desired length. For example, if these lengths tend to be on the order of 24 to 30 feet, then a group comprising at least 48 to 60 linear feet of wallboard must be scrapped or otherwise wasted every time that a cover paper splice occurs. Full manual operation of the knife during the occasional passing of a cover paper splice is not only impractical, but also very difficult to practice with any reliable level of precision due to the high speeds at which the wallboard travels and the knife operates.
Accordingly, there exists a need for an apparatus and method that reduces the amount of wallboard that is scrapped or wasted during the manufacturing process, and in particular the amount of wallboard that is scrapped or wasted due to the cover paper splices that are inherent to the manufacturing process.